Cantilever Bridges
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Can You Believe Cantilever Bridges It? Caisson workers can suffer he cantilever bridge was invented in the from an illness that usually nineteenth century when new railways affects divers. If workers leave the high-pressure air in were being built and lots of new bridges a caisson too quickly, bubbles were needed. Compared to beam bridges can form in their blood. It’s and arch bridges, cantilever bridges called decompression sickness. need fewer supports, so they were good Tfor crossing busy rivers. A long cantilever bridge usually has a pair of cantilevers with a beam bridge between them, linking I don’t feel the cantilevers. very well... Ta-da! The Quebec Bridge has One of the first steel the longest cantilever span cantilever bridges, the A Solid Base in the world, at 1,801 feet Forth Rail Bridge (above), (549 m), even though the was built across the Firth A giant chamber called a bridge was built as long of Forth in Scotland in the caisson, open at the bottom, Why It Works ago as 1919. 1880s. is placed on the riverbed two cantilevers. The other or seabed. High-pressure In 1887, three of the men each represent two air inside keeps water out. engineers who designed cantilevers back-to-back. Workers dig out the mud the Forth Rail Bridge Their arms are in tension inside and the caisson sinks showed why it works. The and the sticks below their until it rests on rock. Then it is man in the middle sits on arms are in compression. filled with concrete to form a the beam bridge linking solid support for a bridge. 12 13 Simple Bridges Cross-section Massive Spinning Wire of cable wire Anchors Suspension Bridges Suspension cables are made of thousands of thin The weight of a suspension Wires wires bundled together. bridge tries to pull the ends bundled uspension bridges became popular The Golden Gate Bridge’s together of its suspension cables out of suspension cables are into cable the ground, so they have to in the twentieth century because they about 3.3 feet (1 m) thick. be locked in place by massive could span longer distances than Each cable is made of blocks of rock or concrete Some suspension bridges are cantilever bridges without lots of 27,572 pencil-thin wires. called anchorages. In Japan, still made in the same way Stringing the wires from each of the Akashi Kaikyo supports, or piers, underneath. They as ancient rope bridges, but one end of the bridge to Bridge’s anchorages weighs using modern materials. The were ideal for spanning wide rivers the other is a process called 390,000 tons. Capilano Suspension Bridge Sand bays where it would be difficult to build a cable spinning. in Canada is a steel cable line of piers. The bridge is held up from above suspension footbridge. Walkers use it to cross the Capilano by vertical cables called hangers, or suspenders. River through the treetops in a The suspenders hang from the bridge’s main tourist park in Vancouver. suspension cables, which are anchored at each end of the bridge. One of the The first modern Can You Believe It? most famous suspension bridges is suspension bridge is the the Golden Gate Bridge across the Menai Bridge between feet The Akashi Kaikyo Bridge in Japan is 3.3 entrance to San Francisco Bay Wales and the island of The biggest bridges are Anglesey in England. It (1 m) longer than planned. While it was being so long that they are in California. opened in 1826 and is still built, an earthquake moved its towers farther affected by the curved in use today. apart. The deck was made a little longer to fill shape of Earth’s surface. the gap. The tops of their towers are farther apart than their bases. 14 15 Strong Shapes Can You strong shape. Steel girders Believe It? Building Bridges to Last The steel plates and girders linked together in triangles that make up parts of bridges combine strength and light The secret ingredient that One of the strangest ngineers build bridges to last by are often linked together in weight, and let the wind blow made Roman concrete so materials used to build a way that makes triangles, through the structure. tough and long-lasting was bridges is plastic water designing them to withstand the because a triangle is a very volcanic ash. The ash was bottles. In 2014, a bridge forces that try to destroy them. The called pozzolana. It helped made of 104,500 bottles different parts of a bridge are built bind the concrete together was built across the very strongly, and it resisted Bega River in Romania. from different materials. The materials damage from seawater. are carefully chosen to resist the Ecompression or tension forces acting on each This bridge must part. One of the oldest bridges in the world that be strong—it’s got Maybe we should is still in use today is China’s Anji Bridge, also lots of triangles! leave before the known as the Zhaozhou Bridge. It is a stone volcano erupts? arch bridge that was completed in the year 605 CE. It has survived ten floods, eight wars, A triangle is strong because Tough Stuff each of its sides locks the and numerous earthquakes. other two sides into position. Reinforced concrete is often used to build bridges. It’s made of concrete with steel rods called rebars embedded in it. Concrete is good at The first major bridge resisting compression, and made of cast iron was built Hinges steel is good at resisting across the River Severn tension (stretching). Together, Iron and steel expand near Coalbrookdale in when they warm up and they make a very strong England in the 1770s. It building material. shrink again when they still stands today. cool down. The top of the Sydney Harbour Bridge can rise or fall by up to 7 inches (18 centimeters) because of temperature changes. Giant hinges at the ends of the arches let the bridge do this without breaking up. 16 17.